Until the 1950's, uranium was generally recovered from leach solutions by precipitation of the uranium. Precipitation methods for isolating uranium were abandoned since quantitative precipitation was not achieved, costs were high, and recirculation of solutions was necessary to prevent high uranium losses. One such method involved reducing with iron and precipitating uranium from sulfuric acid leach solutions as uranous phosphate at a pH between 1.0 and 2.8. A variation of that method involved adding copper sulfate and phosphoric acid to the leach solution and then either adding iron powder or percolating the solution through iron scrap. See R. C. Merritt, The Extractive Metallurgy of Uranium, Colorado School of Mines, pages 223-227 (1971). Because of numerous disadvantages of these precipitation methods, ion exchange techniques developed in the 1950's replaced the precipitation methods for isolating uranium. Instead of precipitating the uranium in the leachate, the leachate was passed over an ion exchange column where uranium ions became bound, and thus concentrated. The uranium was then eluted from the column.
Ion exchange columns are now very commonly used to isolate uranium from leachates obtained from the treatment of insoluble uranium deposits, such as those in subterranean formations, with a carbonate-oxidant leach solution. The oxidant in the carbonate-oxidant leachant, e.g., oxygen, hydrogen peroxide, sodium chlorate or sodium hypochlorite, oxidizes uranium from the insoluble tetravalent state to the soluble hexavalent state according to the overall reaction scheme EQU UO.sub.2 +[O]+H.sub.2 O.fwdarw.UO.sub.2.sup.+2 +2OH.sup.-.
The carbonate in the carbonate-oxidant leachant, obtained for example from ammonium carbonate, sodium bicarbonate, sodium carbonate, carbon dioxide and sodium hydroxide, carbon dioxide and water, or mixtures of the foregoing, combines with uranium in the hexavalent state to form stable and soluble uranyl carbonate complexes, one of which is produced according to the equation EQU UO.sub.2.sup.+2 +3CO.sub.2.sup.-2 .fwdarw.UO.sub.2 (CO.sub.3).sub.3.sup.-4.
The resulting solutions of soluble uranyl carbonate complexes are passed through the ion exchange column. Uranium is eluted from the column and may then be converted to the yellowcake form.
One problem in using ion exchange for concentrating and isolating uranium arises from the presence in the leachate of minerals other than uranium which compete with uranium in the ion exchange process for sites on the ion exchange resin. One such mineral, sodium chloride, when present in the leachate in large amounts, makes uranium concentration and isolation on the ion exchange column difficult if not impossible.
Another problem in using ion exchange resins in concentrating uranium is the presence in the leachate of strongly absorbed substances which resist normal elution and interfere with the loading of uranium. These materials are called resin "poisons" and include silica, molybdenum, polythioate, sulfur and organics. See R. C. Merritt, The Extractive Metallurgy of Uranium, Colorado School of Mines, pages 163-166 (1971).